In recent years, pluripotent stem cells such as ES cells and iPS cells (herein, “ES cells” and “iPS cells” are collectively referred to as “pluripotent stem cells”) have been artificially created and much contribution is expected in the field of regenerative medicine. Pluripotent stem cells have universal differentiability, i.e., being differentiable into various types of cells constituting a living body; therefore, the skin, cartilage, bone, blood vessel, nerve, or organs of a patient can be regenerated without causing rejection by using the patient's own iPS cells.
Since pluripotent stem cells have universal differentiability as described above, there may be cases in which apart of pluripotent stem cells may start differentiating during culturing. The cells which have started to differentiate cannot be reverted to an undifferentiated state and can no longer be used for creating internal organs and organum as aimed. Therefore, during the subculture of pluripotent stem cells, it is important to selectively subculture only undifferentiated pluripotent stem cells.
Such discrimination of the undifferentiated pluripotent stem cells can be carried out by observing cells after being stained or under fluorescent light, using, for example, “CellCelector” manufactured by “AVISO”. However, cell staining is often performed on the cells after being fixed, in general, and dyes are often toxic to cells; therefore, it is difficult to observe the cells while they are still alive. Even the use of a dye with low toxicity is still harmful to the cells and unsuitable for the application in the field of regenerative medicine.